The current global emphasis on environmental protection and pollution abatement has fostered the development of a number of new technologies and processes focused on pollution control in the power production industry. Some of these new technologies represent significant improvements over the previous technologies in the areas of increased pollutant removal efficiencies, reduced reagent requirements, reduced waste streams and reduced operating costs.
One such system is comprised of a selective catalytic reduction (SCR) for the control of nitrogen oxides (NO.sub.x) and sulfuric acid production for sulfur oxide (SO.sub.2) removal. This system results in high efficiency NO.sub.x and SO.sub.2 removal, minimal particulate emissions and no liquid or solid waste production. The system basically begins with some sort of particulate removal apparatus such as an electrostatic precipitator or appropriate filtering equipment. This is followed by a NO.sub.x reduction system which involves the selective catalytic reduction of the NO.sub.x to molecular nitrogen and water by reaction with ammonia (NH.sub.3) and oxygen. Following the selective catalytic reduction of the NO.sub.x, the process catalytically converts sulfur dioxide (SO.sub.2) to sulfur trioxide (SO.sub.3). The SO.sub.3 -containing flue gas stream is then cooled in a wet sulfuric acid condenser, by heat exchange with an air stream, to produce a concentrated sulfuric acid by-product stream. The cleaned flue gas is then ready for release to the atmosphere.
This process generates a considerable amount of recoverable heat in several ways. All of the reactions which take place with respect to NO.sub.x and SO.sub.2 removal are exothermic and increase the temperature of the flue gas. These include the reaction of NO.sub.x and NH.sub.3, SO.sub.2 oxidation, SO.sub.3 hydration to form sulfuric acid fume and condensation of the sulfuric acid. These heats of reaction plus any support heat which may have been added are recovered in the wet sulfuric acid condenser by cooling air. This heated air stream has typically been used for combustion air. A small percentage of the hot air has been used for system auxiliaries, such as ammonia evaporation and dilution, any support burner combustion air required and coal milling. However, the amount of air that is required for cooling in the wet sulfuric acid condenser is usually equal to or greater than the amount required for combustion and auxiliary purposes and/or is often at a higher temperature than desired for either the combustion air preheater or the primary air to the coal pulverizers.